1. Field of the Invention
The present invention relates to an optical line terminal (OLT) and, more particularly, to an OLT that stores the identity numbers of a number of optical network terminals (ONTs) that are associated with a single network end point.
2. Description of the Related Art
The access leg of a communications network is the segment of the network that connects a central office to the end users. A passive optical network (PON) is an access leg that optically transmits data between two points, and requires no power for the data to pass between the two points. Active optical networks also optically transmit data between two points, but require power to pass the data between the two points.
One common architecture of the access leg of a passive as well as an active optical network is a point-to-multipoint architecture. In a point-to-multipoint architecture, an optical line terminal (OLT) in the central office transmits information to, and receives information from, a number of optical devices, such as optical network terminals (ONTs).
There are a number of types of point-to-multipoint architectures used in the access leg of a network that differ by where the ends of the optical fibers of the network (the network end points) are located. For example, a Fiber to the Cabinet (FTTCab) access leg is a point-to-multipoint architecture where the end points of the optical fibers are located in a cabinet in a neighborhood. Existing copper lines are typically used to make the network connection from the cabinet to the end users.
In addition, a Fiber to the Building/Curb (FTTB/C) access leg is a point-to-multipoint architecture where the end points of the optical fibers are located at the curb or in the building. As before, existing copper lines are normally used to make the network connection from the curb or building to the end users.
A Fiber to the Home (FTTH) access leg is a point-to-multipoint architecture where the end points of the optical fibers extend to the end users, such as being connected to the exterior wall of an end user's residence or office. The FTTH access leg provides a continuous optical connection between the OLT and the end users.
In addition to the architecture, optical networks are also commonly described by the type of protocol used on the network or the services provided by the network. For example, an FTTH PON which utilizes the asynchronous transfer mechanism (ATM) protocol is often referred to as an APON or an ATM-PON.
APONs use a single fiber to pass two light frequencies. The two frequencies include a downstream light frequency that is utilized to transmit voice, data, video, and network signals from the OLT to the end users in an ATM format, and an upstream light frequency that is utilized to transmit voice, data, and network signals from the end users to the OLT in an ATM format.
Other variations include an Ethernet PON (EPON) that utilizes the Ethernet protocol, a broadband PON (BPON) which is an extension of an APON, and a gigabit PON (GPON) that combines aspects of an EPON and a BPON (utilizes the ATM protocol as well as non-ATM protocols, such as Ethernet and video protocols). A BPON utilizes two light frequencies to transmit and receive voice, data, and network signals in an ATM format, and a third light frequency to transmit video signals downstream to the end users in a non-ATM format (as an overlay).
FIG. 1 shows a block diagram that illustrates the access leg of a prior art point-to-multipoint BPON 100. As shown in FIG. 1, BPON 100, which can be implemented in accordance with the specifications stated in International Telecommunications Union (ITU) Recommendation G.983.1, which is hereby incorporated by reference, includes an optical line terminal (OLT) 110.
OLT 110 includes an optical transmitter 112, an optical receiver 114, and a wave division multiplexed (WDM) combiner 116. Optical transmitter 112 receives first downstream information, such as voice, data, and network signals, and transmits a sequence of first downstream light pulses that represents the first downstream information to combiner 116 which, in turn, outputs the first downstream light pulses. Optical transmitter 112 transmits the first downstream light pulses to combiner 116 at a wavelength in the range of, for example, 1480-1500 nm.
Combiner 116 receives a sequence of upstream light pulses, and outputs the upstream light pulses to optical receiver 114 which, in turn, converts the upstream light pulses into upstream information, such as voice, data, and network signals. Optical receiver 114 receives the sequence of upstream light pulses from combiner 116 at a wavelength in the range of, for example, 1260-1360 nm.
In addition, OLT 110 includes a controller 120 that is connected to optical transmitter 112 and optical receiver 114. As shown in FIG. 1, controller 120 includes a memory 120A that has a large number of memory cells that store software and data. The software includes an operating system and a set of program instructions. The operating system can be implemented with, for example, the Linux operating system, although other operating systems can alternately be used.
Controller 120 further includes a central processing unit (CPU) 120B that is connected to memory 120A. CPU 120B, which can be implemented with, for example, a 32-bit processor, executes the program instructions to operate on and transform the data. CPU 120B prepares the downstream information for optical transmitter 112, and receives the upstream information from optical receiver 114.
In addition, controller 120 includes a display system 120C that is connected to CPU 120B. Display system 120C, which can be remotely located or portable, allows images to be displayed to system personnel which are necessary for the system personnel to interact with the program. Controller 120 also includes an input system 120D which is connected to CPU 120B. Input system 120D, which also can be remotely located or portable, allows the system personnel to interact with the program.
Further, controller 120 includes a network interface card (NIC) 120E which is connected to memory 120A and CPU 120B. NIC 120E provides a connection to a networked computer to transfer information, such as status information, out of controller 120, and to transfer information, such as program instructions, into controller 120.
As further shown in FIG. 1, BPON 100 additionally includes a WMD combiner 122 that is connected to combiner 116, and a fiber optic cable 124 that is connected to combiner 122. Combiner 122 combines a sequence of transmitted light pulses that represent (overlaid) video signals with the sequence of light pulses output from transmitter 112 via combiner 116. The light pulses representing the video signals can be transmitted at a wavelength in the range of, for example, 1550-1560 nm. In the example shown in FIG. 1, OLT 110 and combiner 122 reside in a central office 126.
As further shown in FIG. 1, BPON 100 also includes a passive optical splitter/combiner 132 that is connected to fiber optic cable 124, and a series of optical fibers OF1-OFn that are connected to splitter/combiner 132. Splitter 132 splits the downstream light pulses into a sequence of split downstream light pulses that are output to the series of optical fibers OF1-OFn. Combiner 132, on the other hand, combines a series of end user upstream light pulses from the series of optical fibers OF1-OFn to output the upstream light pulses.
BPON 100 further includes a series of network end points EP1-EPn which are located at the end users, and a series of optical network terminals ONT1-ONTr that are connected to the series of optical fibers OF1-OFn at the end points EP1-EPn to provide service to the end users.
Each optical network terminal ONT receives the split downstream light pulses from a fiber OF, converts the split downstream light pulses into first local downstream signals, such as voice, data, and network signals, and second local downstream signals, such as video signals. Each optical network terminal ONT also transmits a number of end user upstream light pulses, depending on the voice, data, and network signals output by the end user.
As further shown in FIG. 1, memory 120A maintains relationally-related information, such as in table 134, on the network end points EP1-EPn. For example, memory 120A has a number of memory cells that store the active identity numbers, such as the serial numbers, of the optical network terminals ONT1-ONTr that are connected to the network end points EP1-EPn to provide service to the end users. The first downstream information output by controller 120 includes the active identity number of an optical network terminal ONT when the ONT is to be added to network 100, and when the ONT is connected to network 100.
In addition, memory 120A has a number of memory cells that stores the ranges to the optical network terminals ONTs, e.g., the distances from the OLT to the optical network terminals ONT1-ONTr, and the calculated transmission delays that have been assigned to the optical network terminals ONT1-ONTr. Other information associated with the network end point EP is also held by memory 120A such as, for example, the passwords associated with the active identity numbers, the connection type, and the level of service.
In operation, when a network end point EP is to be added to network 100, the active identity number of the optical network terminal ONT to be connected to the network end point EP to provide service to the end user is added to table 134 in a manner that establishes a relationship between the network end point EP and the active identity number of the ONT.
Controller 120 periodically outputs an identity number message that includes the active identity number of the to-be-added optical network terminal ONT. The identity number message is output onto network 100 to determine if the to-be-added optical network terminal ONT has come on line. For example, in the G.983.1 recommendation, the physical level operations, administration, and maintenance (PLOAM) ATM cells are utilized to determine if a to-be-added ONT has come on line.
When the to-be-added optical network terminal ONT is physically connected to network 100, the added optical network terminal ONT receives the identity number message, and responds to OLT 110 within the allowed time period. (A maximum period of time is allowed for an optical network terminal ONT to respond when the active identity number is output to network 100.)
Once controller 120 of OLT 110 receives the response from the added optical network terminal ONT confirming that an optical network terminal ONT with that active identity number is on the network, controller 120 determines a range to the added optical network terminal ONT.
Controller 120 determines the range by outputting a range message to the added optical network terminal ONT. The range message (which can be the same as the identity number message) includes the active identity number of the added optical network terminal ONT.
The added optical network terminal ONT receives the range message, and responds to OLT 110. The elapsed time between when the range message was sent until when a response is received from the added optical network terminal ONT is measured by controller 120. The range can then be determined knowing the average response time of an optical network terminal ONT, and the speed of the light pulses through optical fiber.
After the range to the added optical network terminal ONT has been determined, controller 120 of OLT 110 determines the transmission delay for the added optical network terminal ONT. Transmission delays are utilized to compensate for the varying distances that ONTs lie from OLT 110.
In the downstream direction (from OLT 110 to the optical network terminals ONT1-ONTr), the split downstream light pulses are received by each optical network terminal ONT. However, in the upstream direction, a time division multiple access (TDMA) protocol is utilized to avoid collisions.
With a TDMA protocol, the upstream time payload is divided into a large number of time slots which are shared by the optical network terminals ONT1-ONTr. An optical network terminal ONT receives a grant message from controller 120 in a time slot that grants the optical network terminal ONT permission to transmit. If each ONT transmitted light pulses after receiving permission, collisions could occur because the optical network terminals ONT1-ONTr typically lie a different distance away from OLT 110, and therefore have a different response time.
To prevent this from happening while increasing the available bandwidth, controller 120 calculates a transmission delay for each optical network terminal ONT based on the range of the ONT. When an optical network terminal ONT receives permission to transmit light pulses onto the network, the optical network terminal ONT must wait the transmission delay period from when the permission was received to begin transmitting.
The transmission delay allows controller 120 to account for the difference in distance-based response times by synchronizing or scheduling the end user upstream light pulses from the optical network terminals ONT1-ONTr so that the end user upstream light pulses from one optical network terminal ONT do not interfere with the end user upstream light pulses from another optical network terminal ONT.
A number of factors, such as temperature, can cause the characteristics of fiber optic cable 124 and fibers OF1-OFn to vary, thereby causing the timing of network 100 to vary. As a result, controller 120 periodically sends the range message to detect changes in the elapsed time, and adjusts the transmission delay as required.